Chitosan is a linear biopolymer with many interesting properties like biocompatibility, non-toxicity, biodegradability, including antimicrobial activity. In recent years, it has been shown that chemical modification of chitosan can lead to significant improvement of its antimicrobial effect. The aim of this study was therefore, to selectively modify the amino group of chitosan by various cationic and lipophilic moieties to obtain derivatives and conjugates having improved antimicrobial activity than chitosan itself.
The synthesis was performed by using 3,6-O-di-tertiarybutyldimethylsilyl protected chitosan as a precursor and the various modifications like trimethylation, guanidinylation, quaternisation and multiple functionalization was carried out at the 2-amino position of chitosan in a controlled manner. The structural modification of the chitosan derivatives was confirmed using FT-IR, 1H-NMR and 2D-NMR spectroscopy, and their average molecular weight was measured by size exclusion chromatography.
Initial assessment of these conjugates towards a panel of clinically important bacterial strains like Staphylococcus aureus (S. aureus, MRSA and MSSA), Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis and clinical A. baumannii isolate showed bactericidal effect at an optimized ratio of the quaternary ammoniumyl group and the lipophilic functionality. We observed that presence of higher degree of substitution and shorter hydrophobic alkyl chains significantly improved the antimicrobial activity, whereas the introduction of spacers between the functional group and the polymer backbone caused a significant reduction in the activity. Simultaneously, we could also control the toxicity of the derivatives by slight alterations in the ratio of the attached moieties. Such polymer conjugates having tunable antimicrobial properties can emerge as promising antibacterial agents. The excellent activity of some of the chitosan derivatives towards planktonic cells led us to further explore their efficacies towards bacterial biofilms. We have recently investigated how the combination of different functional groups influenced chitosan’s efficacy against preformed S. aureus biofilms. The antibiofilm effect of the cationic chitosan derivatives was greatly enhanced in presence of hydrophobic groups (alkyl chains), and the extent of their effect was determined by the ratio and length of the alkyl chains. Living and dead cells were visualized by fluorescence staining, and three-dimensional imaging of biofilms confirmed the accessibility and antimicrobial effect of chitosan derivatives with alkyl chains in the full depth of the biofilms.
We combined the above results to develop an overall structure-activity relationship for these polymers towards planktonic bacteria and bacterial biofilms. Further exploration into the control of biofilms of different bacterial strains utilizing such modified biopolymers are in progress.
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